"Because it was an
allophone of /n/ before final [Ng] was simplified to [N], presumably related
to the reduction of final [mb] to [m] (as in 'lamb') and vacillation in the
reduction of final [nd], which allowed 'parasitic' [d] as in English _sound_
from Old French from Latin _sonus_." Richard WORDINGHAM,
between /mb/ > /m/ and /Ng/ > /N/ is very good.
/nd/ > /n/
occurs at least in one word : "handsome".